1. Field of the Invention
The present invention relates to a method of manufacturing a stator yoke of a rotary electric machine.
2. Description of the Related Art
Japanese Patent application JP-A-64-60247 discloses a method of manufacturing a yoke. In the method, a sheet made of magnetic material, such as iron or steel, is rolled and a number of convexities formed at one side thereof are fitted to the same number of concavities. Thereafter, respective central portions of the convexities of one side of the steel sheet are punched and expanded, thereby clamping the opposite sides of the steel sheet. It is also known that the convexities are shaped into dovetail convexities.
However, because the convexities on a side of a steel sheet are expanded and deformed while the concavities on the other side are not expanded, differences in shape and thickness arise between the one side and the other side. This may cause the circularity of the yoke to be inaccurate. If the circularity is not accurate, it is not possible for a rotary electric machine to provide a small air gap between the rotor and the stator thereof. If the air gap is not made small, the performance of a rotary electric machine lowers.
As a conventional technology of manufacturing a stator yoke of a rotary electric machine, there is a method of manufacturing a yoke disclosed in JP-A-5-91700. In this method, a sheet member is rolled to meet the opposite sides of the sheet member together, and the opposite sides are welded together. For this purpose, positioning concavities are formed on the surface of the sheet member. However, an expensive laser beam has to be used to weld the opposite sides.
It is also known that dovetail convexities are formed at one side of the sheet member, which are fitted and clamped to concavities formed at the other side of the sheet member. In this case, sealant is filled in the clamped portions to ensure the seal, as disclosed in JP-A-52-20207. However, extra steps of filling sealant and removing leftover sealant are necessary, and work time and production cost increase.
Further, a yoke of a motor, such as a starter motor, is manufactured from a steel plate, which is rolled up to form a cylindrical shape so that the opposite sides of the plate are put together and welded to each other as disclosed in JP-U-58-97957 and JP-A-64-60247. There is a chamfered corner at the inside of the yoke in order to have a bracket or the like fitted in the open end of the yoke smoothly.
Such chamfered corner of the yoke is formed by a machine after the steel plate is rolled up. Therefore, the manufacturing cost is high.